The performance and cost of light emitting diodes (LEDs) have improved to the point where they are now replacing incandescent, fluorescent, compact fluorescent, and halogen lamps in lighting applications. LEDs have many advantages over other light sources including long life, ruggedness, low power consumption, and small size. LEDs are near-monochromatic light sources, and are available with emission peaks from the UV through the visible and into the infrared. A typical bandwidth for an LED is less than about 50 nm. Accordingly, in order to simulate white light, it is necessary to generate a broader range of wavelengths than is emitted by a single LED. There are two common solutions to this problem. One approach is to arrange individual red, green, and blue (R, G, B) LEDs in close proximity and then diffuse and/or mix the light emitted by groups of at least three LEDs. A second approach is to combine a short-wavelength (UV or blue) LED with broadband fluorescent compounds or phosphors that convert part or all of the LED light into longer wavelengths.
Using separate R, G, B light emitting devices introduces several problems. The different LED colors are typically provided by LEDs made of different semiconductor materials, requiring different operating voltages and complex drive circuitry. The resulting emission spectrum consists of three relatively narrow color bands, and the result is a poor approximation of natural daylight (approximately a broadband blackbody radiation that arrives at the Earth surface from the Sun). Color rendering can be compromised due to metamerism effects where color appearance can vary according to the illumination wavelengths.
While white light fixtures and devices using RGB LEDs have been offered commercially, the single-LED-plus-phosphor solution has generally emerged as the preferred way to use LEDs for general white-lighting applications. Only a single type of LED (UV or blue) is required together with one or more fluorescent or phosphorescent materials. Typical phosphors can efficiently convert at least a portion of the incident light into relatively broadband emission at longer wavelengths. The converted light typically has peak emission in the green to yellow with a bandwidth of at least 100-150 nm. A typical device is configured to mix some unconverted blue light with broadband yellow-green converted light to give a much better approximation of blackbody white light than can be obtained using RGB LEDs. The overall device (one LED plus phosphor) can be more compact, simpler in construction, and lower in cost than the 3-LED device, and the color rendering can be superior as well.
Various materials are available commercially for use as phosphors for solid state lighting. The phosphors are generally inorganic powders with up to a few percent of a rare earth material such as cerium or europium in a base crystalline material. Common base materials include garnets, silicates, nitrides, and sulfides. The doped material is typically ground into a micron-sized powder for use in lighting applications.
Commercial devices use one or several mixed phosphors to convert a portion of blue LED emission to green, yellow, orange, and red. Some of the blue light from the LED is transmitted through the phosphors and mixed with the yellow phosphor emission, thereby resulting in a perceived white light.
Phosphors can be prepared by mixing the ground material with a polymer such as an epoxy or silicone to create a wet slurry that can be dispensed, painted, or printed on the surface of the LED. Alternatively, though less commonly, the wet can be coated onto a “remote” phosphor plate (at some distance from the LED) or used to create a molded plastic part that incorporates phosphor particles.